This study includes a 10-day morphine taper phase, followed by two study phases (approximately 18 days each) with daily active ibudilast and placebo administration, respectively. After the detoxification phase, participants are randomized to receive placebo or MN-166, and then be stabilized on the medication. Thereafter, participants will complete laboratory sessions. Subsequently, during Phase 2, participants will cross over to the other treatment arm, stabilize, and complete laboratory sessions.